Not absolutely freeware available, but you can try OST to PST Converter trial version which has the same functionality and features as full software version. The tool provides facility to save 20 emails per folder for free after converting inaccessible OST email data. It offers many alternative saving options except PST format such as MSG, EML, RTF, HTML, or PDF formats. The software also provides sorting and organization facility for converting email data on the basis of To, From, Date, Subject, etc. You can get a copy of this OST to PST Converter tool, follow this https://www.datarepairtools.com/ost-...converter.html

Though there's no freeware tool available for OST to PST conversion, but there are few easy manual tricks which you can use here. By using the manual tricks, you can easily transfer mailbox content from OST files to new PST files. There's no need to use any 3rd party for that. All you need to use is MS Outlook client, that's it. Outlook client has few built-in features, such as Archive, Export to a new PST file, Move the items to a new PST file and Drag & Drop. Refer to this blog post to know how to use these features - Convert OST to PST Format using 3 Easy Manual Tricks | N. Sem's Blog

Before you can use any of the manual tricks shared in the above blog post, make sure:

You are using the same computer system on which the OST files were originally created.

Your Outlook profile is connected with MS Exchange Server.

And most importantly, the OST files are not corrupt.

You can't use any of the manual tricks if your OST files are corrupt. In such situation, you should try a professional converter utility. Download OST Recovery software that works as a recovery cum converter utility. First it'll extract mailbox content from OST files (normal as well as corrupt), and then save them into new PST files. Other than PST format, you can save OST files' content into DBX, EML, MSG, MBOX and archive (EML, VCF, ICS and text).

Try manual methods for OST to PST Conversion, It's provide reliable and efficient solution for offline storage OST file to equivalent personal storage PST file. I always preferred you first try manual methods. If you are searching a third party OST to PST Converter tool, then I suggest first try the demo version of the tool. Purchase only if the demo version works.
Source Link:- Convert OST file into PST format.